Black patches on meshes that use transparency
One thing I know for certain is that this problem is getting less bad as I raise the number of transparency light bounces

You've answered your own question right there.  To avoid getting hung up tracing rays that will never reach a light source, Cycles has a maximum number of bounces for each ray.  If a ray reaches that maximum number without hitting any light source, Cycles assumes it never will and renders it black.
I am assuming your scene uses planes with a partially-transparent grass texture on them, this results in many rays which have to pass through a large number of the transparent parts of the planes.  If your transparent bounces are low, this results in black patches.
If you want to render your scene as-is then, there's no way of getting around raising the max transparent bounces until the black patches are gone.
If you want to change the way the grass is made, you can get around this issue by modeling grass objects and using a particle system or other method of instancing them across the ground.  This avoids the rays having to collide with a bunch of 100% transparent faces.  If you provide some images or a .blend file I may be able to give a similar example.